This invention relates to electron emissive coatings for thermionic cathodes. More particularly, it relates to such cathodes for arc discharge lamps. Still more particularly, it relates to such coatings having a lowered work function and thus lowered lamp starting voltages and increased lamp efficacy.
Thermionic cathodes are employed as the electron source in many applications, including arc discharge light sources such as fluorescent lamps. For many years these cathodes have used an emissive material coated upon a tungsten or similar coil, which is heated by the passage therethrough of an electric current. The emissive material has been applied as the carbonates of barium, calcium, strontium and, occasionally, zirconium oxide. This material is subsequently subjected to thermal breakdown during lamp processing, whereby the carbonates are decomposed to the respective oxides.
The life of a fluorescent lamp is determined primarily by the evaporative life of the cathode coating. The vapor pressure of barium oxide as a function of temperature is described by the following equation:
log10Pmm=xe2x88x92(19,700/T)+8.87
where T is the temperature in Kelvins. Since the rate of evaporation is such a strongly temperature dependent function even rather modest changes in cathode operating temperature can have a profound effect on lamp life.
It would be an advance in the art if this emissive material could be changed to provide an even lower work function, which in the case of fluorescent lamps, would result in lower lamp discharge voltage with a concomitant increase in lamp efficacy, reduced cathode hot spot temperature, a reduction in lamp starting voltage, and an increase in life.
It is, therefore, an object of this invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the operation of thermionic cathodes.
Yet another object of the invention is an improved fluorescent lamp.
These objects are accomplished, in one aspect of the invention, by the provision of an electron emissive coating for a thermionic cathode that comprises the oxides of barium, calcium, strontium and optionally zirconium and an effective amount of silicon carbide to increase the electron emissivity of said coating over that of a similar coating without the silicon carbide.
These objects are further accomplished by the provision of a thermionic cathode that comprises a tungsten coil and an electron emissive coating on the tungsten coil. The coating comprises the oxides of barium, calcium, strontium and optionally zirconium and an effective amount of silicon carbide to increase the electron emissivity of the coating over that of a similar coating without the silicon carbide.
The objects are still further accomplished by the provision of an arc discharge lamp that comprises an evacuated, electromagnetic-energy-transmissive envelope; an arc generating and sustaining medium within the envelope; and at least one thermionic, electron-emitting cathode within the envelope, the cathode having an electron emissive coating thereon containing silicon carbide.
The use of the invention described herein results in a reduction in work function, a lowering of cathode voltages and a longer life for lamps in which they are employed.